1. Field of the Invention
The present invention relates to a process for the recovering and reusing organic binder components from the lacquer sludge accumulating during the spraying of solvent-containing, one- or two-component polyurethane lacquers.
2. Description of the Prior Art
Two-component polyurethane lacquers containing solvent have by virtue of their outstanding characteristics acquired great practical importance in the coating industry. In practice, the lacquers are as a rule applied by spray onto the substrates to be coated. With this process it is not possible to avoid considerable amounts of the coating agent employed being lost as a result of overspray. In practice these excess amounts are generally washed out of the exhaust air of the coating plant with the aid of water and are caused to coagulate by the use of coagulating agents, for example, surface-active, inorganic material such as alumina. Previously, it has not been possible to re-use this "lacquer sludge."
An object of the present invention is to develop a method which enables useful processing of the binder components present in the lacquer sludge before these components become totally unusable as a result of the steadily progressing crosslinking reaction. The process should also be applicable to solvent-containing, one-component polyurethane lacquers, wherein the binders are NCO prepolymers which undergo curing in the presence of moisture.
It has now been found that that this objective may be achieved such that lacquers containing
a) organic polyisocyanates, PA1 b) preferably, in the case of two-component polyurethane lacquers, organic compounds with groups capable of reacting with isocyanate groups, preferably organic polyhydroxyl compounds, PA1 c) optionally pigments and other additives and PA1 d) solvent, can be recovered and reused if the overspray of the lacquer is caused to react with compounds which i) have been dissolved or dispersed in the water of the coating plant, optionally by the use of emulsifiers, and ii) contain groups which are more reactive with isocyanate groups than the reaction partners of the polyisocyanates and water. The lacquer sludge is isolated by the use of coagulating agents and the organic constituents are extracted. PA1 i) washing the overspray out of the exhaust air of a coating plant with the aid of an aqueous medium which a) contains in dissolved or dispersed form compounds that exhibit greater reactivity towards isocyanate groups than either water or the isocyanate-reactive groups present in two-component polyurethane lacquers, and b) may contain emulsifiers, PA1 ii) isolating the chemically modified overspray formed in step (i) in the form of lacquer sludge by treatment with coagulating agents, Which may be present in the aqueous medium, PA1 iii) mixing the lacquer sludge, containing fully reacted lacquer constituents, coagulating agents and water, optionally after a portion of the water is removed, with an organic solvent and PA1 iv) separating the organic phase accumulating in step iii) from the aqueous phase and insoluble constituents which may be present and recovering the solution of modified organic binder components dissolved in the organic phase.